Senate Minority Leader Chuck Schumer told reporters he didn't see a reason to withdraw from the agreement.

"There are no reports that Iran has violated the agreement," he said. "To me, the greatest worries from Iran are not right now (on) the nuclear side, but rather what they're doing in Syria."

Dick Durbin, the No. 2 Democrat in the Senate, called the withdrawal a "mistake of historic proportions."

"The last thing America and the world need right now is a new nuclear threat. Breaking this deal increases the danger that Iran will restart its nuclear weapons program, which threatens our ally, Israel, and destabilizes the entire Middle East," he said in a statement. "It isolates the United States from the world at a time when we need our allies to come together to address nuclear threats elsewhere, particularly in Korea. This is a mistake of historic proportions."

Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, said in a statement that such a move drives "a wedge between us and our allies."

"Simply withdrawing the United States from the JCPOA will not benefit the American people and US national security: it will only succeed in driving a wedge between us and our allies, whose help we need to enforce any future sanctions regime against Iran, and will effectively green light Iran's pursuit of nuclear weapons," he said in the statement. "Withdrawing from this agreement makes the United States, and the world, less secure."

And House Minority Leader Nancy Pelosi called Tuesday a "sad day" because of the announcement.

"Democrats have no illusions about the Iranian regime. We remain strongly committed to stopping the advancement of Iran's ballistic missile program, its egregious human rights abuses, and its support of terrorism and other nefarious activities in the region. Today is a sad day for America's global leadership," she said in a statement. "The Trump Administration's dangerous and impulsive action is no substitute for real global leadership."

Just before the announcement, Democratic members prepared themselves for reports that Trump was planning to begin exiting the deal.

While members cautioned they wanted to know exactly what Trump would do before commenting, Bob Menendez, the ranking member of the Senate Foreign Relations Committee, said it was "a huge mistake to withdraw without a plan."

"I think it's a huge mistake to break away without important allies and I think it is another mistake to have no strategy to deal with all of Iran's nefarious activities especially without allies," Menendez said.

Menendez warned that Congress needed to consider intervening if it could.

"That's something we need to think about," Menendez said.

Sen. Tim Kaine, another member of the Senate Foreign Relations Committee, told CNN that it would be problematic if Trump renewed sanctions.

"I think he will be driving the wedge between the United States and our European allies," Kaine said.

While most Republicans have consistently opposed the Iran deal -- a key accomplishment of President Barack Obama -- some Republicans had been encouraging Trump not to pull out of the agreement without a plan B.

"Without proof that Iran is in violation of the agreement, it is a mistake to fully withdraw from this deal," he said in a statement.

Over the weekend, House Armed Services Chairman Mac Thornberry told Fox News he didn't want to see the US pull out completely from the Iran deal.

Sen. Susan Collins, a Republican from Maine, also said the Iran deal had been flawed, but she preferred an approach where the U..S would "remedy those flaws" with allies rather than walking away all together.

Senate Foreign Relations Committee Chairman Bob Corker said it was disappointing the White House couldn't reach an agreement with US allies, but is confident the President will work for a better deal.

"Based on conversations I have had in recent days, it is my sense that the administration will move quickly to work toward a better deal," he said in a statement. "Moving forward, I will continue to work with the administration, my colleagues in Congress, and our foreign partners on a policy that actually meets our shared goal: preventing Iran from being able to produce a nuclear weapon."

Arizona Republican Sen. Jeff Flake, a frequent Trump critic, told CNN's Jake Tapper that he doesn't think Trump's decision is "a wise move," and that he does not believe the country is safer as a result of the action.

"I just don't think that it's a wise move," Flake said in an interview on "The Lead with Jake Tapper." "Our allies and our adversaries need to know that we are reliable, and I think that's missing right now."